FIG. 1 is a diagram of a network architecture of UMTS (universal mobile telecommunications system) of an asynchronous IMT-2000 system.
Referring to FIG. 1, a universal mobile telecommunications system (hereinafter abbreviated UMTS) mainly includes a user equipment (hereinafter abbreviated UE), a UMTS terrestrial radio access network (hereinafter abbreviated UTRAN) and a core network (hereinafter abbreviated CN).
The UTRAN includes at least one radio network sub-system (hereinafter abbreviated RNS). And, the RNS includes one radio network controller (hereinafter abbreviated RNC) and at least one base station (hereinafter called Node B) managed by the RNC. And, at least one or more cells exist in one Node B.
FIG. 2 is an architectural diagram of a radio protocol used in UMTS.
Referring to FIG. 2, a plurality of radio protocol layers exist as pairs in UE and UTRAN, respectively to take charge of a data transmission in a radio section. Each of the radio protocol layers is explained as follows.
First of all, a physical layer (PHY) as Layer 1 plays a role in transmitting data in a radio section using various radio transmission schemes. The physical layer (PHY) is connected to a MAC layer as an upper layer via a transport channel. And, the transport channels can be classified into a dedicated transport channel and a common transport channel according to a presence or non-presence of a channel sharing.
In Layer 2, MAC (medium access control), RLC (radio link control), PDCP (packet data convergence protocol) and BMC (broadcast/multicast control) layers exist. The MAC layer plays a role as mapping various logical channels to various transport channels and as logical channel multiplexing for mapping several logical channels to one transport channel. The MAC layer is connected to the RLC layer as an upper layer via a logical channel. And, the logical channels are mainly classified into a control channel for transferring information of a control plane and a traffic channel for transferring information of a user plane according to a type of information to be transported.
The MAC layer is divided into a MAC-b sublayer, a MAC-d sublayer, a MAC-c/sh sublayer, a MAC-hs sublayer and a MAC-e sublayer according to a type of a transport channel managed in detail.
The MAC-b sublayer takes charge of management of a transport channel BCH (broadcast channel) responsible for a broadcast of system information. The MAC-c/sh sublayer manages such a common transport channel shared with other UEs as FACH (forward access channel), DSCH (downlink shared channel) and the like. The MAC-d sublayer takes charge of managing DCH (dedicated channel) as a dedicated transport channel for a specific UE. The MAC-hs sublayer manages HS-DSCH (high speed downlink shared channel) as a transport channel for high speed downlink data transmission to support high speed data transmission in downlink or uplink. And, the MAC-e sublayer manages E-DCH (enhanced dedicated channel) as a transport channel for high speed uplink data transmission.
The RLC layer takes charge of securing a quality of service (hereinafter abbreviated QoS) of each radio bearer (hereinafter abbreviated RB) and a corresponding data transmission. An RLC places one or two independent RLC entities in each RB to secure a generic QoS of the corresponding RB and offers three kinds of RLC modes including TM (transparent mode), UM (unacknowledged mode) and AM (acknowledged mode) to support various QoS. And, the RLC plays a role in adjusting a data size to be suitable for a lower layer to transfer data in a radio section. For this, the RLC generates PDU (protocol data unit) by segmenting and concatenating SDU (service data unit) data received from an upper layer and then delivers the PDU to a lower layer.
The PDCP layer is placed above the RLC layer and enables data, which is transferred using such an IP packet as IPv4 or IPv6, to be efficiently transferred in a radio section having a relatively small bandwidth. For this, the PDCP layer performs header compression which is to raise transport efficiency of a radio section by transferring necessary information via header of data only. Since header compression is a basic function of the PDCP layer, the PDCP layer exists in a packet service (PS) domain only. And, one PDCP entity exists per RB to provide effective header compression to each packet service (PS).
In Layer 2, the BMC (broadcast/multicast control) layer exists above the RLC layer. The BMC layer schedules a cell broadcast message and performs a function of broadcasting on UEs located within a specific cell.
A radio resource control (RRC) layer placed on a lowest part of Layer 3 is defined on the control plane only. The RRC layer, which is associated with configuration, reconfiguration and release of RBs, controls parameters of Layer 1 and Layer 2 and takes charge of controlling logical, transport and physical channels. In this case, the RB means a logical path provided by Layer 1 and Layer 2 of a radio protocol for a data delivery between UE and UTRAN. Generally, ‘configuring RB’ means that characteristics of a radio protocol layer and channel necessary for providing a specific service are regulated and that each detailed parameter and operational method are established.
The RLC layer is explained in detail as follows.
First of all, basic functions of the RLC layer are a guarantee for QoS of each RB and a corresponding data transmission. Since an RB service is a service Layer 2 of a radio protocol provides to a higher layer, the whole parts of Layer 2 affect the QoS. Specifically, the RLC considerably affects the QoS. The RLC leaves an independent RLC entity on each RB to guarantee generic QoS of the corresponding RB. And, the RLC provides three kinds of RLC modes such as a transparent mode (hereinafter abbreviated TM), an unacknowledged mode (hereinafter abbreviated UM) and an acknowledged mode (hereinafter abbreviated AM) to support various kind of QoS. Each of the three modes of the RLC supports a different QoS. So, the three modes of the RLC differ from each other in an operational method and in a detailed function. Hence, the RLC needs to be taken into consideration in aspect of its operational mode.
The transparent mode (TM) is a mode that no overhead is attached to RLC SDU delivered from an upper layer in configuring RLC PDU. Namely, since the RLC lets SDU pass transparently, it is called TM RLC. And, the TM RLC performs the following roles in user and control planes. In the user plane, since a data processing time is relatively short within RLC, the TM RLC takes charge of transmission of real-time circuit data such as voice or streaming in a circuit service domain (hereinafter abbreviated CS domain). In the control plane, since there is no overhead within RLC, the TM RLC takes charge of a transmission for an RRC message from an unspecific UE in case of uplink or a transmission for an RRC message broadcast to all UEs within a cell in case of downlink.
Unlike TM, a mode that an overhead is attached by RLC is called a non-transparent mode. And, the non-transparent mode is classified into a UM (unacknowledged mode) having no acknowledgement for the transmitted data and an AM (acknowledged mode) for the transmitted data. UM RLC sends PDUs by attaching a PDU header including a sequence number (hereinafter abbreviated SN) to each of the PDUs so that a receiving side can know what PDU is lost in the course of transmission. In a user plane, Owing to this function, the UM RLC mainly takes charge of a transmission of broadcast/multicast data or real-time packet data such as voice (e.g., VoIP) and streaming in a packet service domain (hereinafter abbreviated PS domain). In a control domain, the UM RLC takes charge of a transmission of an RRC message requiring no acknowledgement among RRC message transmitted to a specific UE or a specific UE group within a cell.
AM RLC configures PDU by attaching a PDU header including SN like the UM RLC. Yet, the AM RLC makes a great difference from the UM RLC in that a receiving side makes an acknowledgement for PDU transmitted by a transmitting side. The receiving side makes the acknowledgement in the AM RLC, which is because the receiving side makes a request for a transmitting side's retransmission of the PDU failing in being received by the receiving side. And, this retransmission function is an outstanding feature of the AM RLC. So, the object of the AM RLC is to guarantee an error-free data transmission via the retransmission. And, the AM RLC mainly takes charge of a transmission of non-real-time packet data such as TCP/IP of the PS domain in the user plane. In the control plane, the AM RLC takes charge of a broadcast of an RRC message necessarily requiring acknowledgement among RRC messages transmitted to a specific UE within a cell.
In aspect of directionality, TM RLC and UM RLC are used for uni-directional communications. On the other hand, AM RLC is used for bi-directional communications sue to feedback from a receiving side. Since the bi-directional communications are mainly used for point-to-point communications, the AM RLC uses a dedicated logical channel only. In structural aspect, one RLC entity of TM or UM RLC is constructed with one structure of transmission or reception, whereas both transmitting and receiving sides exist within one RLC entity of the AM RLC.
The complication of the AM RLC is attributed to the retransmission function. For the retransmission management, the AM RLC needs a retransmission buffer as well as a transceiver buffer. And, the AM RLC performs various functions such as a use of transmitting/receiving window for a flow control, polling that a transmitting side makes a request for status information to a receiving side of a peer RLC entity, a status report that a receiving side makes a report of its buffer status to a transmitting side of a peer RLC entity, status PDU for carrying status information, a piggyback of inserting status PDU within data PDU to raise efficiency of data transmission, etc. Besides, there is a reset PDU that makes a request for resets of all actions and parameters to AM RLC entity of the other side in case that AM RLC entity discovers crucial error in an operational process or a reset ack PDU used for an acknowledgement of the reset PDU. To support theses functions, the AM RLC needs various protocol parameters, status parameters and a timer. PDU used for the control of data transmission in AM RLC such as status report or status PDU, reset PDU and the like is called control PDU. And, PDU used in delivering user data is called data PDU.
In brief, PDU used by AM RLC can be mainly classified into two types. A first type is data PDU and a second type is control PDU. The control PDU can be classified into four types including status PDU, piggybacked status PDU, reset PDU and reset ack PDU.
As mentioned in the foregoing description, one of the cases of using control PDU is a reset procedure. The reset procedure is used in solving an erroneous situation in an operation of AM RLC. For instance, the reset procedure is used in solving a situation that mutually used sequence numbers are different from each other or that PDU or SDU fails in transmission over a predetermined count. If the reset procedure is used, AM RLC of a receiving side and AM RLC of a transmitting side reset environmental variables to enter a state for resuming communications.
The reset procedure is executed in a following manner.
First of all, AM RLC of a transmitting side includes a currently used HFN (hyper frame number) value in a transmitting direction in a reset PDU and then transmits the reset PDU to a receiving side.
In case of receiving the reset PDU, AM RLC of a receiving side resets a HFN value in its receiving direction and initializes environmental variables such as a sequence number and the like.
And, the AM RLC of the receiving side includes its HFN in its transmitting direction in a reset ack PDU and then transmits the reset ack PDU to the AM RLC of the transmitting side.
Once receiving the reset ack PDU, the AM RLC of the transmitting side resets the HFN value in its receiving direction and then initializes environmental variables.
FIG. 3 is a structural diagram of AM RLC PDU (AMD PDU) as data PDU that is used in transmitting data.
Referring to FIG. 3, AM RLC PDU is used in case that AM ELC entity attempts to transmit user data, piggybacked status information or polling bit. A user data part is constructed by an integer multiplication of 8 bits. And, a header of the AM RLC PDU is constructed with a sequence number having a 2-octet size. Moreover, a header part of the AM RLC PDU includes a length indicator (LI).
FIG. 4 is a structural diagram of status PDU.
Referring to FIG. 4, a status PDU includes different kinds of super fields (SUFIs). A size of the status PDU is variable but is limited to a size of a biggest RLC PDU of a logical channel carrying the status PDU. In this case, the SUFI plays a role as information indicating what kind of AM RLC PDU arrives at a receiving side or what kind of AM RLC does not arrive at the receiving side. The SUFI is constructed with three parts of type, length and value.
FIG. 5 is a structural diagram of piggybacked status PDU.
Referring to FIG. 5, a structure of a piggybacked status PDU is similar to that of a status PDU. The piggybacked status PDU differs from the status PDU in that a D/C field is replaced by a reserved bit (R2).
The piggybacked status PDU is inserted in AM RLC PDU in case that a sufficient space remains. And, a PDU type value (type_) is always fixed to ‘000’.
FIG. 6 is a structural diagram of reset/reset ack PDU.
Referring to FIG. 6, a reset PDU includes a sequence number of 1-bit RSN.
And, a reset ack PDU is transmitted in response to a received reset PDU and is transmitted by including the RSN included in the received reset PDU.
Parameters used in the above PDU formats are explained in detail as follows.
1) D/C Field: D/C filed is a filed indicating whether a corresponding PDU is a control PDU or a data PDU.
2) PDU Type: PDU type indicates a type of a control PDU. IN particular, the PDU type indicates whether a corresponding PDU is a reset PDU or a status PDU.
3) Sequence Number: This value means sequence number information of AM RLC PDU.
4) Polling Bit (P): This value is set in case that a request for a status report is made to a receiving side.
5) Extension Bit (E): This value indicates whether a next octet is a length indicator or not.
6) Reserved bit (R1): This value is used for a reset PDU or a reset ack PDU and is coded into 000.
7) Header Extension Bit (HE): This value indicates whether a next octet is a length indicator or data.
8) Length Indicator: This value indicates a location of a boundary in case that a boundary between different PDUs exists within a data part of PDU.
9) PAD: This part is a padding area that is not used for AM RLC PDU.
As mentioned in the foregoing description, a status PDU corresponds to a case that control information and padding information are included within one AMD PDU (AM data PDU). And, a piggybacked status PDU means control information when user data and control information are placed within one AMD PDU. A format of the piggybacked status PDU is substantially identical to that of the status PDU. Yet, these PDUs are classified according to how one AMD PDU is filled.
Since the piggybacked status PDU or the status PDU is not user data, they should be minimally transmitted to raise efficiency in aspect of data transmission. Yet, in data transmission of AM RLC, since a transmitting side always needs acknowledgement indicating that data is correctly received from a receiving side, it is unable to completely reduce the transmission of the status PDU or the piggybacked status PDU.
In An RLC operation according to a related art, AM RLC preferentially transmits control information in case that there are control information and user data to be transmitted. In transmitting the control information, a UE is unable to use the piggybacked status PDU in case that there is no spare space in AMD PDU. Hence, the UE transmits the control information using the status PDU. In this case, a transmission of the AMD PDU configured in advance may be delayed.
FIG. 7 is an exemplary diagram for explaining an operation of AM RLC according to a related art.
Referring to FIG. 7, it is assumed that maximum two PDUs can be transmitted during one TTI transmit time interval) and that AM RLC has sufficient data to be transmitted. So, it is assumed that the AM RLC lies in a situation that there are data enough to fill up AM RLC PDU.
Since AM RLC has no control information to be transmitted during TTI 1, AMD PDU including user data only is configured and transmitted. During TTI 2, in case that there is control information to be transmitted, the AM RLC has to transmit the control information to AM RLC of the other side using status PDU or piggybacked status PDU. Yet, since it is assumed that the AM RLC has sufficient user data to be transmitted, a padding bit cannot be generated no matter how the AMD PDU is configured. So, the piggybacked status PDU cannot be included. Hence, the AM RLC has to transmit the control information using the status PDU. In this case, on the assumption that the AM RLC can use maximum twp AMD PDUs during one TTI, the AM RLC transmits one AMD PDU including user data only and one status PDU during TTI 2. And, AMD PDU including user data only like TTI 1 is transmitted during TTI 3.
However, the related art has the following problems.
First of all, in spite that there is sufficient data to be transmitted by AM RLC, in case that a size of status report is smaller than that of AMD PDU, a padding bit having no significance as information, as shown in FIG. 7, exists in a status PDU. This means a serious transmission efficiency reduction in case that there are excessive user data to be transmitted by the AM RLC.
As mentioned in the foregoing description, in using the piggybacked status PDU rather than the status PDU, a portion making a configuration of PDU inefficient like a padding bit is reduced. Yet, in the related art, the piggybacked status PDU is not facilitated to use. The reason is explained as follows. First of all, in case that there are control information and user data to be transmitted, AM RLC preferentially fills AM PDU with the user data and then inserts the control information in a spare space of the AMD PDU in a format of a piggybacked status PDU. Meanwhile, in the related art, in case that a piggybacked status PDU is included in a specific AMD PDU, a portion or whole portions of SDU included right prior to the piggybacked status PDU should correspond to a last portion of the SDU. In particular, a first portion of a AMD PDU transmitted next to the AMD PDU including the piggybacked status PDU starts as a first portion of a new SDU.
FIG. 8 is a diagram for explaining a method of configuring AMD PDU including piggybacked status information according to a related art.
Referring to FIG. 8, in case that user data and control information to be transmitted exist, AM RLC-insets SDU 1 and SDU 2 in an nth AMD PDU and then decides whether there is a space in which the control information can be inserted in a format of a piggybacked status PDU into the nth AMD PDU. If it is decided that the control information can be inserted into the nth AMD PDU, the piggybacked status PDU is inserted. In doing so, the SDU 2 located ahead of the piggybacked status PDU should be ended in front of the piggybacked status PDU and cannot be included in an (n+1)th AMD PDU. In other words, if the SDU is segmented into at least two portions, it is unable to include one portion in the nth AMD PDU and the other portion in the (n+1)th AMD PDU.
However, in the related art, a size of a piggybacked status PDU is limited to a space remaining after AMD PDU is filled up with at least one SDU. So, even if AM RLC needs to use the piggybacked status PDU, it is frequently unable to use the corresponding PDU. If control information to be transmitted to a receiving side is generated, it is highly probable that the AM RLC may use status PDU rather than the piggybacked status PDU which is more advantageous in aspect of transmission efficiency. Hence, the transmission efficiency of the control information becomes is further lowered.